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NEW Grey Zucchinni Not Doing Well. WHY?!?

My new grey zucchinni was doing beautifully 6 weeks ago but now the larger of the leaves are turrning yellow/brown and wilting. This is also spreading to my other zucchinnis and cucumber plants. They are getting plenty of water, and sunlight, and there are no known bugs or eggs on the leaves, besides ants. I NEED HELP! And soon before this spreads to my baby pumpkin and watermelon plants! Please help before it's too late!